Aliphatic polycarbonates are a sort of polymers with good mechanical properties, which are widely applied in realms such as food packaging, plastic thin films, etc. Meanwhile, aliphatic polycarbonates likewise have superior biocompatibility and biodegradation. Particularly functional materials can be obtained by improving aliphatic polycarbonates, which have been comprehensively used in aspects such as medicine carriers, tissue engineering materials, medical sutures, medical screws and the like. As a result, the aliphatic polycarbonates always are the focus attracting attention of chemists.
Accounts of Chemical Research 37(2004) 836 and Angew Chem Int Ed 43(2004)3574 reported that Mn of aliphatic polycarbonates prepared by copolymerizing CO2/epoxides with [Cr(salen)] and [Co(salen)] as catalysts respectively can be up to 8.9×103 and 3.04×104. As the catalysts can hardly be segregated thoroughly, a great deal of Co2+ and Cr2+ remain in the polymers. The types of metallic ions are severely noxious, and the application of the polymers will bring huge safety risks; moreover, zinc glutaric acid is regarded as a kind of catalysts with the most enormous potential in industry. Macromolecules 35(2002)6494 discovered that catalytic efficiencies of the kind of catalysts mostly are relatively low, and each gram of the catalysts merely can generate tens of or less than ten grams of products. Therefore, consumption of the catalysts has to be increased to achieve mass production of products, which raises costs of the products significantly and affects performance of the products; Chinese patents CN 101440159A and CN 1583825A published the method of preparing aliphatic polycarbonates with transition metal cyanides as the catalyst, but the heavy metal ions in the sort of catalyst can hardly be segregated, and the residual catalyst will degrade the quality of the polymers. Molecular weights of the polymers also are relatively low, which are not fit for used as plastic directly; a patent CN 102197063A disclosed a method of removing metal complexes as catalysts from aliphatic polycarbonates, but processes of the method are too complex to reduce the costs of polymers.
Adopting melt transesterification to synthesize polymers has advantages such as simply processes, high catalytic efficiencies and environmentally safety. Therefore, the melt transesterification recently receives broad attention from APCs researchers. American Cross research group (Macromolecular 2007, 40:7934) utilized the diethyl carbonate and 1,4-butylene glycol to synthesize APCs by melt transesterification with a lipase as the catalyst, but the maximal weight-average molecular weight Mw of the prepared polymer is less than 6.0×104. A patent CN 101643542A and the paper Polym. Int., 60(2011)1060 employed TiO2/SiO2(PVP) as the catalyst to prepare APCs by a novel method of melt transesterification, and the number-average molecular weight of the prepared polymer tested by GPC can be up to 1.8×105, which can meet the requirement of the number-average molecular weight of plastic grade APCs, and the value is 7.0×104. U.S. Pat. No. 8,168,728 B2 used titanium tetrabutoxide as the catalyst in the melt transesterification reaction of methyl carbonates and diols to prepare the polycarbonate. The content of terminal hydroxyl in the polymer prepared by the method is high and the molecular weight is low, which cannot be applied in plastic industries directly. A patent JPH08143656 reported a preparation method of aliphatic polycarbonate with high molecular weight. The method used zinc acetate dihydrate and zirconium acetylacetonate as the catalysts to synthesize block APCs polymers whose number-average molecular weight Mn is larger than 15000 by the transesterification reaction of diphenyl carbonates, aliphatic dicarboxylic ester and diols. Recently, U.S. Pat. No. 9,447,234 B2 reported to catalyze dimethyl carbonates and diols with sodium methoxide by the melt transesterification reaction to synthesize the APCs new catalysis system. The weight-average molecular weight Mw of obtained polymers can be up to 2.48×105.
It can be learnt from the methods above that molecular weights of polymers obtained by the melt transesterification catalyzed by bio-enzyme generally are relatively low, which cannot be applied as plastic directly; but in other acid-base catalysis systems, catalysts and products can hardly be separated thoroughly. The remaining catalysts will affect colors and thermostability of the product, as well as generating toxicity out of the product. Nowadays, a method of dissolving polymers in chloroform and filtering to segregate catalysts is commonly used in basic research papers. The adoption of the method obviously will generate a great amount of poisonous waste, which cannot satisfy the environmental request for mass production, and the method cannot thoroughly separate the catalysts from the product.